1. Field of the Invention
The present invention relates to a process for fine modelling of the ground clutter received by a radar.
2. Discussion of the Background
For the purpose of evaluating performance, radar specialists have long sought to simulate the radar echoes which will be returned by the ground ("clutter") when the radar is deployed and put into service at its location of use, without having to wait for the radar actually to be installed there.
Since the characteristics of the clutter are highly dependent on the location of installation, it is important to be able accurately to model the ground clutter with which the radar will have to cope in the places where it is installed.
Since there are no maps giving the characteristics of clutter over the entire Earth and in all frequency bands and under all polarizations, the most appropriate starting data are numerical terrain models. These cover practically the whole planet and give, in digitized form, at every point of a grid of reasonably fine spacing, the altitude and the nature of the ground, as well as an indication of the nature of the elements present on it (forests, roads, rivers, structures, etc.). They are the starting point of many modern simulations of ground clutter.
Known methods, for example modelling software based on numerical terrain models developed by MIT in the USA, take account, for each terrain element considered, of the angle of illumination, the propagation of the waves between the radar and the relevant location (with, in some cases, allowance for masking effects and reflections) and an estimate of the reflectivity of the ground deduced from the angle of incidence and the category of landscape encountered. The average height of the objects present on the relevant terrain element is used only to calculate the angle of incidence and the propagation attenuation.
At best, the ground clutter is calculated there by determining, from the height of the ground, plus possibly the average height of the objects, the angle of incidence of the waves received from the radar, and by then estimating a radar reflectivity (the ratio of the radar cross section to the unit of physical area). This reflectivity is chosen from among several possible values, the choice being guided by the frequency band used, the polarization, the angle of illumination and the type of landscape (rural, urban, plain, mountain).
The results obtained with the prior art methods exhibit, as compared with the actual reality, sizeable errors whenever the ground bears elements of significant vertical extent. This is because such elements are the major contributors of ground clutter at low, sometimes grazing angles of incidence, and the choice, relatively arbitrary, of a reflectivity value corresponding to the average of a landscape does not take them into account correctly; hence the deviations noted.
This lessens the credibility of the performance predictions deduced therefrom, in particular in the case of surface radars which view the ground at very low angles of incidence, and for which elements present on the ground are the main source of echoes with widely dispersed amplitudes.